1. Field of the Invention
The present invention relates to an optical semiconductor device mainly used for optical communication, and particularly relates to an optical semiconductor device in which quantum dots are used in an active layer.
2. Description of the Related Art
An optical semiconductor device in which quantum dots are used in an active layer enables the realization of a semiconductor element having a small pattern effect and a wide gain band, and therefore its practical use is expected.
It is known that quantum dots can be formed on a substrate in the form of mutually isolated islands by utilizing a so-called S-K (Stranski-Krastanow) mode growth, which appears in the initial phase of heteroepitaxial growth, in a strained heteroepitaxial structure of InAs/GaAs or the like.
An optical amplifier which uses the quantum dots formed by the S-K mode growth has a characteristic of having no gain for so-called TM polarized light and having large polarization dependence. Polarization independence in which the amplification factor of output light is fixed without depending on the polarization state of input light is indispensable for the practical use of the optical amplifier. Therefore, a columnar dot formed by stacking in layers and combining plural quantum dots and combining them into TM mode light is proposed and manufactured by way of trial.
In an optical semiconductor device including columnar dots, an active layer 102 is provided on a semiconductor substrate as shown in FIG. 20. The active layer 102 is constituted by providing a quantum structure 113 between a lower barrier 111 and an upper barrier 112. The quantum structure 113 is composed of columnar dots 121 each formed by stacking in layers, growing, and directly combining plural quantum dots 131 and side barriers 122 each formed by stacking respective side barrier layers 132 corresponding to respective quantum dots 131 so as to be embedded between adjacent columnar dots 121.
In the columnar dot, the luminous efficiency of a TM mode is increased by forming the columnar dot high by increasing the number of stacked layers of quantum dots, thereby obtaining high gain. Namely, in order to increase the luminous intensity ratio of the TM mode, it is necessary to increase the length-to-width ratio of the columnar dot.
However, in the columnar dot, a strain exists therein, and hence there is a problem that by stacking the quantum dots in layers, the crystallinity of the active layer deteriorates and thereby the luminous intensity reduces. For example, in the case of a columnar dot with a size of 20 μm long×15 μm wide, the PL spectral intensity of the columnar dot when the number of stacked layers of quantum dots is 11 (11-fold columnar dot) reduces to about half as compared with a single-layer quantum dot. By increasing the number of stacked layers to obtain a 14-fold columnar dot, polarization independence can be realized. However, in the 14-fold columnar dot, the luminous intensity deteriorates, and thus the level of a crystalline state applicable to the trial manufacture of the optical semiconductor device has not been reached.
Hence, it is required to improve the luminous efficiency of the columnar dot. As a measure against this, there is a method of introducing a tensile strain into the side barrier to relieve the strain accumulated in the columnar dot. It is known that according to this method, the luminous intensity of the TM mode light is increased. It is thought that to realize the polarization-independent optical semiconductor device, it is effective to form the columnar dot by adopting the above-described method.
(Patent Document 1)
Japanese Patent Application Laid-open No. 2004-111710
(Patent Document 2)
Japanese Patent Application Laid-open No. 2003-197900
(Patent Document 3)
Japanese Patent Application Laid-open No. 2005-72338
However, even if the quantum structure in which the tensile strain is introduced into the side barrier is adopted as described above, the following problem arises.
FIG. 21A and FIG. 21B are characteristic charts each showing an evaluation result as a relation between the number of stacked layers of quantum dots in the columnar dot and PL spectral intensity (luminous intensity of TE mode light−luminous intensity of TM mode light; dB) regarding the polarization dependence of the columnar dot. Here, FIG. 21A shows a case where the side barrier has no strain, and FIG. 21B shows a case where a compressive strain of −0.5% (namely, a tensile strain of 0.5%) is introduced into the side barrier, and FIG. 21A and FIG. 21B both show cases where the wavelengths of output light are 1450 nm, 1500 nm, and 1550 nm, respectively.
As shown in FIG. 21A and FIG. 21B, it is found that if the number of stacked layers of quantum dots in the columnar dot is increased, the luminous intensity of the TM mode light gradually increases. Here, as in FIG. 21A, in the columnar dot when the side barrier has no strain, the luminous intensity of the TM mode light increases by about 1 db every time the number of stacked layers of quantum dots increases by one. As in FIG. 21B, as concerns the effect of the introduction of the strain into the side barrier, the luminance intensity of the TM mode light increases by about 2 dB when there is a strain of −0.5% in the 7-fold columnar dot.
However, as shown in FIG. 21B, in the columnar dot when the strain is introduced into the side barrier, there is a problem that even if the length-to-width ratio is increased by increasing the number of stacked layers, the increasing rate of the luminous intensity of the TM mode light is small so that a target set value cannot be obtained. As compared to FIG. 21A, the increasing rate of the luminous intensity of the TM mode light is about ¼.